title: Altered architecture of substrate binding region defines the unique specificity of UDP-GalNAc 4-epimerases.
abstract: UDP-hexose 4-epimerases play a pivotal role in lipopolysaccharide (LPS) biosynthesis and Leloir pathway. These epimerases are classified into three groups based on whether they recognize nonacetylated UDP-hexoses (Group 1), both N-acetylated and nonacetylated UDP-hexoses (Group 2) or only N-acetylated UDP-hexoses (Group 3). Although the catalysis has been investigated extensively, yet a definitive model rationalizing the substrate specificity of all the three groups on a common platform is largely lacking. In this work, we present the crystal structure of WbgU, a novel UDP-hexose 4-epimerase that belongs to the Group 3. WbgU is involved in biosynthetic pathway of the unusual glycan 2-deoxy-L-altruronic acid that is found in the LPS of the pathogen Pleisomonas shigelloides. A model that defines its substrate specificity is proposed on the basis of the active site architecture. Representatives from all the three groups are then compared to rationalize their substrate specificity. This investigation reveals that the Group 3 active site architecture is markedly different from the "conserved scaffold" of the Group 1 and the Group 2 epimerases and highlights the interactions potentially responsible for the origin of specificity of the Group 3 epimerases toward N-acetylated hexoses. This study provides a platform for further engineering of the UDP-hexose 4-epimerases, leads to a deeper understanding of the LPS biosynthesis and carbohydrate recognition by proteins. It may also have implications in development of novel antibiotics and more economic synthesis of UDP-GalNAc and downstream products such as carbohydrate based vaccines.
authors.affiliation: Department of Biochemistry, The Ohio State University, Columbus, Ohio 43210, USA.
authors.forename: Veer S
authors.initials: VS
authors.lastname: Bhatt
authors.forename: Chu-yueh
authors.initials: CY
authors.lastname: Guo
authors.forename: Wanyi
authors.initials: W
authors.lastname: Guan
authors.forename: Guohui
authors.initials: G
authors.lastname: Zhao
authors.forename: Wen
authors.initials: W
authors.lastname: Yi
authors.forename: Zhi-Jie
authors.initials: ZJ
authors.lastname: Liu
authors.forename: Peng G
authors.initials: PG
authors.lastname: Wang
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002238:Carbohydrate Epimerases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure; D016981:Plesiomonas / Q000201:enzymology*; D011135:Polysaccharides, Bacterial / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013379:Substrate Specificity; D014574:Uronic Acids / Q000737:chemistry / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D011135:Polysaccharides, Bacterial; D011994:Recombinant Proteins; D014574:Uronic Acids; C102528:altruronic acid; D002238:Carbohydrate Epimerases; C559984:UDP-N-acetylgalactosamine 4-epimerase, Pleisomonas shigelloides; C023223:UDP-N-acetylglucosamine 4-epimerase